1. Field of the Invention
The present invention relates to turbine engines, and more particularly, to turbine engines having improved cooled and pressurized air distribution downstream of the impeller.
2. Description of the Prior Art
It is known to bleed off high pressure air, known as P3 air, from the diffuser of the compressor into the impeller rear cavity, downstream of the centrifugal impeller for the purpose of loading the impeller and thus removing stress on the adjacent bearings. However, such pressurized air is generally at high temperature and tends to degrade the impeller hub, thereby reducing the effective life of the impeller. It is also noted that the impeller bearings must be sealed from this high temperature air, thus adding weight to the engine.
In order to reduce the degradation effect on the impeller, it has been contemplated to use high heat resistant materials. However, this will only increase the cost of the impeller while providing a weaker impeller hub more susceptible to fractionating under high centrifugal forces.
The central bearing, located downstream of the impeller, adjacent the impeller rear cavity, is sometimes cooled by compressor bleed air which has been cooled through an external heat exchanger using bypass air as a coolant. This air, known as P2X air, is at a lower pressure, thus requiring additional seals to isolate the P2X air and the central bearing from the P3 air in the impeller rear cavity that surrounds the central bearing. The P3 air is at a relatively high temperature T3.